Re-Os Decoupling at the Subgrain-scale by Quantitative Compositional Mapping in a Molybdenite from the Altar Porphyry Deposit, San Juan, Argentina. Implications on the associated Re-Os Age.

VALLEJOS, MARTÍN GONZALO; MAYDAGAN, LAURA; SELBY, DAVID

Bahia Blanca

Congreso; 14° Congreso de Mineralogía, Petrología Ígnea y Metamórfica, y Metalogénesis (14º MinMet y 5º PIMMA); 2023

Universidad Nacional del Sur - Asociación Geológica Argentina

The Re-Os (rhenium-osmium) chronometer applied to molybdenite has become a powerful tool that permits the determination of precipitation ages of this mineral with very high precision via isotope dilution-negative thermal ionization mass spectrometry (ID-NTIMS). However, the widely known spatial decoupling between Re (parent isotope) and 187Os (daughter isotope) within individual molybdenite grains results in a number of considerations and establishes analytical protocols that must be followed to successfully apply this technique and obtain reproducible ages (Stein et al, 2003; Selby and Creaser, 2004). In addition, it is essential that the samples, consisting of fully homogenized molybdenite separates, derive from grains precipitated from a single hydrothermal pulse, and therefore have not been affected by late overgrowths, which are difficult to detect optically. Otherwise, the ages obtained would represent an average between the ages of each hydrothermal pulse.The Altar porphyry Cu-(Au-Mo) deposit (San Juan, Argentina) comprises an extensive stockwork system with hydrothermal veinlets related to mineralization that occur to depths up to two thousand meters in all the mineralized bodies of the district: Altar East, Altar Central and Quebrada de la Mina-Radio. This system includes molybdenite-bearing veinlets with variable amounts of quartz and other sulfides (mainly pyrite and chalcopyrite). Maydagán et al. (2020) reported a Re-Os molybdenite age of a veinlet from Altar East area, which yields 11.16 ± 0.06 Ma with ~3,546 ppm of Re.Here we present quantitative wavelength-dispersive spectrometry (WDS) X-ray mapping performed in a single molybdenite crystal from the same sample previously dated, that was also selected due to its high Re content which favours its detection and visualization. The aim of this study is to determine the single or multiepisodic nature of the molybdenite (which could have implications in Re-Os ages) and corroborate the existence of spatial decoupling between Re and Os at the subgrain-scale. WDS maps of Re, Os, Mo, S, and Cu in the molybdenite crystal were carried out using a Cameca SX100 electron microprobe at the Center for Advanced Materials Characterization in Oregon (CAMCOR) from the University of Oregon, which is equipped with 5 tunable wavelength dispersive spectrometers. The operating conditions were 40 degrees take-off angle, beam energy of 22 keV, beam current of 50 nA, and beam diameter of 0 μm (focused beam).The maps show that, within the analysed molybdenite, Re and Os are heterogeneously distributed, whilst Mo and S are homogeneously distributed. Rhenium presents an oscillatory zoning in parallel and slightly concentric bands (Fig. 1a), clearly defining both a Re-rich (7,000-18,000 ppm) and a relatively Re-poor (2,500-7,000 ppm) sectors. Osmium (2,200-3,800 ppm) was only detected in a sector matching with the Re-poor sector mentioned above (Fig. 1b), which suggests a spatial decoupling between Re and Os within the molybdenite crystal (outside the crystal, Os shows homogeneous distribution; Fig. 1b). As for molybdenum, it is evenly distributed in concentrations that vary between 52-62% (Fig. 1c).The results of this study would confirm that the analysed molybdenite was generated from a single hydrothermal pulse, which is evidenced by the oscillatory zoning of Re along the whole crystal. Any secondary overgrowth, could have generated sharp changes in the Re distribution, as was recognized in other WDS mapping studies (Barra et al., 2017). Thus, it can be suggested that the age obtained by Maydagán et al (2020) represents a single event based on the monoepisodic nature of this molybdenite crystal. Regarding the distribution of Re and Os, this study presents the first compositional mapping of a molybdenite crystal where the spatial decoupling between them is evident, which is in agreement with previous interpretations based on LA-MC-ICP-MS analyses as well as experimental studies where different protocols to the ID-NTIMS technique were applied (Stein et al, 2003; Selby and Creaser 2004). The mechanism by which this decoupling occurs is still a matter of debate, although it has been suggested that it could be related to the migration of 187Os by diffusion towards crystal dislocations and defects (Stein et al, 2003).Finally, we suggest that mappings similar to those here presented should complement geochronological studies that apply the Re-Os chronometer on molybdenite, in order to strengthen the data obtained and thus enhance interpretations, especially when considering the duration of a mineralizing event.